Background
Lightning surges contain
significant amounts of energy that needs to be diverted away
from the "vulnerable equipment" (system that is to be
protected from surges). Electronic equipment, if CE marked, is
designed with a certain built-in surge voltage immunity in
accordance with IEC 1000-4-5—this level of immunity will be
sufficient for the majority of office buildings where the
"vulnerable equipment" is located far away from the
electric service entrance. Thus, equipment that carries the CE
mark will work well without surge protection in many
environments.
However, when the
"vulnerable equipment" is installed close to the
electric service entrance, or in an outdoor enclosure,
particularly in areas with a poor physical ground (dry sand,
rock, etc.), it is often necessary to add external surge
protective devices (SPDs). IEC recommends (IEC 1024) that
lightning protection zones (LPZ) are established to successively
reduce lightning currents, in stages, down to the built-in
immunity of the "vulnerable equipment." The number of
zones required depends on the structure containing the
installation and the "vulnerable equipment." IEC 1024
requires that an SPD is installed at each zone boundary?
the surge current and voltage ratings of individual SPDs must
not be exceeded.
This paper explains the purpose
and concept of surge protective device (SPD) coordination in
accordance with the IEC 1312 standard.
Purpose of Coordination
To successively reduce the
lightning threat, in stages (SPD 1, 2 & 3) down to the surge
withstand capability of the "vulnerable equipment,"
without exceeding surge current and voltage ratings of
individual SPDs.
Variant I
Figure 1
Coordination Concepts
IEC 1312 defines four
coordination "variants." The first three utilize
individual single-port SPDs while the fourth variant is a
two-port hybrid design.
Variant I
Shown in figure 1, the rated
voltage of SPDs is identical and coordination is achieved by
separating each SPD by means of series impedance. Variant I is
not recommended by the IEC, probably because it would require
approximately 30’ (10 m) of wire or separate inductors between
each SPD, which is not practical when installing equipment close
to the electric service entrance, or in outdoor enclosures.
Variant II
Shown in figure 2, the rated
voltages of SPDs are stepped so that the SPD in the
"vulnerable equipment" has the highest rated voltage
and SPD 3, 2 & 1 have progressively lower rated voltages,
thus assuring that each upstream SPD would divert progressively
higher currents. Variant II is difficult to implement as most
power supplies, uninterruptible power systems and telecom
rectifiers, rated for 230VAC, use input SPDs with a 275 Vrms
rating. Thus, stepping down rated voltage, would cause SPD 1 and
2 to be destroyed by normal line voltage
fluctuations—experience has shown that fluctuations of plus
10% resulting in 250V is not unusual.
Figure 2
Variant III
Shown in Figure 3, includes a
component with a non-linear current/voltage characteristic, such
as a spark gap. The SPD 1 spark gap would divert the majority of
the surge current and output a combined wave, similar to
ANSI/IEEE C62.41-1991, Category B3, Combination Wave, 3000A,
6000V, to downstream SPDs 2 and 3. The voltage ratings of SPD 2
and 3 are identical, but each device will handle considerably
less current thanks to the much higher energy handling
capability of spark gap SPD 1. Variant III is a better choice
than I or II as the spark gap is insensitive to fluctuations in
line voltage. However, spark gap follow-on, short-circuit
current may be an issue, and MOVs will still be sensitive to
line voltage fluctuations.
Figure 3
Variant IV
Figure 4 shows a two-port hybrid
device that incorporates cascaded stages of SPDs internally
coordinated with series impedances. A hybrid device can be
designed to maximize performance while reducing the undesirable
characteristics of spark gaps and SPDs based on varistor or
silicon-avalanche-diode technology. The use of a hybrid device
eliminates the need to coordinate surge protective devices, but
does not eliminate sensitivity to prolonged line over voltage.
Figure 4
Summary
Equipment designed to conform to
CE requirements has a built-in surge immunity level that is
sufficient in most office applications. However, equipment
installed close to the electric service entrance or in an
outdoor enclosure, particularly in areas with a large number of
lightning days, requires additional protection—especially if
the physical ground is poor. Surge protective devices require
coordination to perform well. The remnant voltage of the SPD
closest to the "vulnerable equipment," shall not
exceed its built-in immunity level. Coordination of surge
protective devices is not always easy and the best solution may
be a two-port, hybrid device, which will simplify the planning
and installation process. High line voltage will destroy
varistor and silicon-avalanche devices that are rated too close
to the line voltage and must be considered when planning for
protection in areas with unstable power lines.
